In a typical boiling water nuclear reactor, a reactor core includes a core plate supporting a plurality of fuel assemblies within a core shroud. The core shroud provides a barrier to separate upward coolant flow through the core from the downward flow in an annulus surrounding the core within the reactor vessel. The fuel assemblies each comprise a plurality of fuel rods carried on a lower tie plate and supported within a fuel channel. The lower tie plate is received in an opening through the core plate and has an inlet at its lower end for flowing coolant upwardly through the core plate and tie plate into and about the fuel rods within the fuel channel. A bypass flow through the tie plate is also provided for flowing coolant between the channels. Consequently, there is generally an upward flow of coolant from below the reactor core through the tie plate and into and about the fuel assemblies during normal operation of the reactor.
In the event of a drop of coolant level below the top of the active fuel assemblies, or a major loss of coolant in the event of a large recirculation line break that drains the annulus, a core spray sparging system is used to distribute coolant to each fuel assembly to prevent fuel damage. That is, the core spray sparging system prevents fuel rod cladding damage in the event the core becomes uncovered due to loss of coolant. The sparging system includes nozzles mounted in a sparger ring located inside the reactor vessel above the fuel assemblies for directing coolant as a spray over the area of the fuel assemblies. The core spray system also includes pumps, valving, piping and instrumentation necessary to provide coolant to the reactor.
Core spray sparging systems, however, may degrade and break down, resulting in a loss of ability to provide uniform coolant flow across the core. While core spray spargers can be serviced during refueling operations and parts replaced, there is considerable expense involved. Consequently, in the event of a drop of coolant level, for example, due to a recirculation line break, the core spray sparger system may not have the remaining capacity due to degradation or breakdown to achieve a minimum coolant flow to each fuel channel and prevent fuel cladding damage. This is a result of the inability of the core spray spargers when degraded or broken down to provide distribution of sufficient coolant to make up for the flow of coolant out of the core due to backflow leakage through the tie plates and the core plate. For example, upon a large recirculation line break, coolant flows downwardly through the fuel channels and from between the fuel channels through various leakage paths including the lower tie plate bypass holes through the core plate. These backflow leakage paths are designed into the reactor to approximate 10% of normal operating core flow in order to maintain sub-cooling of the water between the fuel channels. That is, approximately 90% of the backflow leakage is through the lower tie plate bypass holes and finger springs mounting the channel. Stated differently, the lower tie plate holes are sized to ensure that 10% of the flow is bypassed in normal operation when flow is moving upwardly through the core plate. Following a large recirculation line break, the flow from between the channels reverses and flows through the bypass holes into the lower tie plate and through the core plate. It will be appreciated that in the event of a loss of coolant flow due to a line break, and a degraded or broken down core sparger system, the core sparger may have insufficient capability to provide adequate coolant distribution to the fuel assemblies to prevent fuel cladding damage.